Electric motor driven traction vehicles, such as trains, trams etc., are known to have two braking devices; a friction braking device comprising, for example, friction blocks or pads applied to the wheels or axles under the control of compressed air or hydraulics, and a dynamic braking device comprising a resistor system which is applied as an electrical load on the traction motor, thereby retarding its rotation.
Dynamic braking resistor systems are known to comprise a number of resistor banks each having a pair of supporting members with a resistor element held therebetween. The element may be constructed from several parts which may be joined together by welding and bent into a zigzag shape. The bent element is held on to the supporting members by clips at each of the bends and flanges at the joins, through which the supporting members extend. An electrical connection is made to each end of the element and insulating spacers isolate the element from the supporting members.
The need to weld parts together increases manufacturing time, and therefore cost, and welds may be liable to failure and may compromise reliability. In addition, there are now available resistor alloys which are cheaper than those previously available, and which have good temperature stability characteristics. However, some of these new alloys are not well adapted to welding.
The clips which are known to be used to hold resistor elements to the supporting members may have lugs which are crimped around the edges of the element. However, these have been found inadequately to hold the element which may, at the very least, slide in the clip, rub against the element edge and work loose.
Resistor banks in traction vehicles may be subject to intense mechanical stresses and vibrations which may, at the very least, cause flexing of the supporting members. Generally, insulating spacers for isolating the element from the supporting members are mounted end to end on each of the supporting members. The engaging faces of adjacent spacers are usually plane. In the event of large-scale flexure of the supporting members, adjacent spacers may effectively part company so that crevices are created therebetween. These crevices are vulnerable to the ingress of containments whose presence may compromise the electrically insulating property of the spacers.
EP-A-0676774 discloses a resistor bank having a first and second supporting members, a resistor element comprising a strip of resistor material which is bent into a zigzag shape and holding means for holding the element at a bend thereof on to a respective one of the supporting members. EP-A-0676774 also discloses insulating spacers for isolating the element from the supporting members.